MUCOUS MEMBRANE. 



401 



intestinal villi,) and their sides tend to assume 

 the direction of radii from a common centre. 

 Hence they are sometimes even triangular in 

 outline. Their opposite or free extremity is 

 much thicker, often as thick as the part bulged 

 by the nucleus, and near this extremity neigh- 

 bouring particles are generally very intimately 

 attached to one another, having often the ap- 

 pearance of being blended into a single mass. 

 The best example of this is on the villi of the 

 small intestine (Jig- 280). The contiguous par- 

 ticles, however, are fitted closely together in 

 the greater portion of their length, and to effect 

 this the bulging nuclei vaiy in the height at 

 which they are placed. There can be no doubt, 

 that, in certain situations at least, as will be 

 afterwards shown, these particles are being con- 

 tinually shed, and consequently are being per- 

 petually renovated. But it is very difficult to 

 ascertain their early condition and changes, and 

 I am not aware of any satisfactory observations 

 having been made for this end. From the 

 great facility with which they become detached 

 from the surface they invest, it is next to im- 

 possible to examine them in situ on thin verti- 



Fig. 230. 



Fig. 282. 



Villus of the intentinum ilium of the Dog, with the 

 epithelium partially detached. 



a a, solitary particles remaining attached j b, club- 

 shaped extremity of the villas from which the 

 epithelium has been detached ; c c, epithelium 

 at its base. Magnified 150 diameters. 



d, detached particles, shewing their close union, 

 especially at the surface (at the letter) ; e, other 

 detached particles, shewing their various shape, 

 their nuclei and nucleoli. The letter is placed 

 at their free extremity. Magnified 350 diameters. 



Fig. 281. 



a, ciliated epithelial particle from the 

 inner surface of the membrana tym- 

 pani of the human subject ; b, cili- 

 ated epithelial particles from the 

 bronchial mucous membrane of the 

 human subject. All those shew the 

 nuclei and nucleoli. Magnified 300 

 diameters. 



Epithelial particles from the cornu uteri of the Cow. 

 The opposite cornu contained a foetus one inch and 

 a half long. 



a, small particle, apparently in an early stage of 

 development. The nucleus is smaller than in the 

 other specimens ; b, another more advanced the 

 nucleus and surrounding substance are both 

 larger, especially the latter, which presents a fine 

 granular texture ; c, a particle made angular by 

 pressure against others. It presents two nuclei, 

 as though formed by fission ; d, another of a dif- 

 ferent shape ; e, detached nucleus, showing its 

 transparency and clear outline ; also two excen- 

 tric dots, the nucleoli. Magnified 300 diameters. 



cal sections. But there is no reason for sup- 

 posing their mode of growth to be originally 

 different from that of the scaly variety. Their 

 nuclei probably appear first on the surface of 

 the basement membrane, and around these a 

 cell is developed (Jig. 280, ). But this cell 

 from its earliest period seems to contain an 

 amorphous substance, which under high micro- 

 scopic powers looks finely mottled, but not so 

 definitely so as to allow of being called granu- 

 lar. As the particle advances towards its full 

 size, it loses its cell-membrane, and when com- 

 plete is to be regarded rather as a solid mass of 

 organic substance, surrounding a nucleus, than 

 as a cell. Here, then, is a striking difference 

 between the scale and the prism : maturity 

 being marked in the one by the disappearance 

 of the substance of the cell ; in the other, by 

 that of the cell-membrane. 



Of tlie nplie?'oidal variety (see Jigs. 273 to 

 277). In this the particles are of a rounded 



Fig. 283. 

 Three epithelial particles from the 



human liver. 



a, nucleus ; b, nucleolus , c 1 , fatty 

 particle. 

 Magnified 300 diameters. 



form, though generally somewhat flattened 

 where they touch. They are always thick, 

 from the substance they contain. It is this 

 variety that constitutes the chief mass of the 

 secreting glands, and hence it might not impro- 

 perly be styled glandular. It corresponds with 

 the prismatic variety, in its usually constitu- 

 ting in the glands a single layer, and in the 

 predominance, from the first, of its substance 

 over its membrane. In the glands, indeed, the 

 membrane can seldom be discerned at all, and 

 the substance surrounding the nucleus, though 

 more bulky, has the same finely mottled cha- 

 racter already noticed in the prisms. In other 

 situations the cell-membrane is persistent, but 

 even then it never flattens into a scale. This 

 variety presents in the different glands nume- 

 rous modifications, which have not yet been 

 studied with the accuracy they merit. It is 



